In a machine tool, when a workpiece finishes machining, the pallet having the machined workpiece mounted thereon is dismounted from a table or pallet mounting table of the machine tool and conveyed to a setup position or other pallet standby station, while another pallet having a setup workpiece mounted thereon is mounted on the table of the machine tool, thereby changing pallets between the table of the machine tool and the pallet standby station. Pallets are changed using an automatic pallet change system or manually by a worker using a crane etc. In each case, for good precision machining, the pallet has to be accurately arranged at a predetermined position on the table or pallet standby station.
For such positioning, various methods may be used. For example, Japanese Patent Publication (B2) No. 8-18216 discloses a method of positioning a pallet and a pallet support table in a pallet transfer system using a stacker crane by making knock pins provided on the pallet support table engage with holes provided in the bottom of the pallet. Further, Japanese Patent No. 3307477 discloses a method of making taper outer peripheral surfaces of front ends of a plurality of piston members provided on a table engage with a plurality of taper holes provided in the bottom of a pallet for rough positioning and then making a plurality of taper cones provided on the table and a plurality of taper holes provided in the bottom of the pallet engage for main positioning.
On the other hand, installation of an automatic pallet change system requires a large space for at least the conveyance of pallets and transfer of pallets. Therefore, when sufficient space cannot be secured, it is not possible to install an automatic pallet change system. Further, an automatic pallet change system requires a complicated mechanism and is high in price, and therefore this is not suited to cases where costs have to be kept down. In this case, a worker manually operates a crane etc. to hoist a pallet from the pallet standby station and move it to above the table and to lower the pallet while arranging the pallet at a predetermined position on the table to thereby change pallets.
In this regard, the positioning method disclosed in Japanese Patent Publication (B2) No. 8-181216 and Japanese Patent No. 3307477 utilizes holes provided in the bottom of the pallets. Therefore, when a worker manually operates a crane to position a pallet, visual recognition of holes is difficult and guiding the holes provided in the bottom of the pallet to the positions of pins on the table requires skill and takes time.